Re: [Qemu-devel] [PATCH v3 01/10] tcg-runtime: add lookup_tb_ptr helper

["Emilio G. Cota" <cota@braap.org>]

On Wed, Apr 26, 2017 at 18:45:31 -0400, Emilio G. Cota wrote:
> On Thu, Apr 27, 2017 at 00:29:49 +0200, Richard Henderson wrote:
> > On 04/26/2017 11:56 PM, Emilio G. Cota wrote:
> > >On Wed, Apr 26, 2017 at 10:40:45 +0200, Richard Henderson wrote:
> > >>On 04/26/2017 08:23 AM, Emilio G. Cota wrote:
> > >(snip)
> > >>>+    cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
> > >>>+    tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(addr)]);
> > >>>+    if (likely(tb && tb->pc == addr && tb->cs_base == cs_base &&
> > >>>+               tb->flags == flags)) {
> > >>
> > >>This comparison is wrong.  It will incorrectly reject a TB for i386 guest
> > >>when CS_BASE != 0.  You really want
> > >>
> > >>   tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(addr)]);
> > >>   if (tb) {
> > >>     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
> > >>     if (tb->pc == pc && tb->cs_base == cs_base && tb->flags == flags) {
> > >>       return tb->tc_ptr;
> > >>     }
> > >>   }
> > >>   return tcg_ctx.code_gen_epilogue;
> > >
> > >wrt the comparison, the only change I notice in your suggested change is
> > >   tb->pc == pc
> > >
> > >instead of
> > >   tb->pc == addr
> > >
> > >, which seems innocuous to me (since tb->pc == addr).
> > >
> > >I fail to see how this relates to your "CS_BASE != 0" comment.
> > >What am I missing?
> > 
> > Recall how you computed vaddr for target/i386:
> > 
> >   addr = pc + cs_base
> 
> I see, thanks!

Hmm TB's are added to tb_jmp_cache by pc, not by pc + cs_base:

  atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);

Shouldn't we then pass just the pc (without adding cs_base) to
lookup_ptr, then? i.e.

--- a/target/i386/translate.c
+++ b/target/i386/translate.c
@@ -2533,11 +2533,7 @@ gen_eob_worker(DisasContext *s, bool inhibit, bool recheck_tf, TCGv jr)
     } else if (s->tf) {
         gen_helper_single_step(cpu_env);
     } else if (!TCGV_IS_UNUSED(jr)) {
-        TCGv vaddr = tcg_temp_new();
-
-        tcg_gen_add_tl(vaddr, jr, cpu_seg_base[R_CS]);
-        tcg_gen_lookup_and_goto_ptr(vaddr);
-        tcg_temp_free(vaddr);
+        tcg_gen_lookup_and_goto_ptr(jr);
     } else {
         tcg_gen_exit_tb(0);
     }

And while at it, rename the "addr" argument in lookup_ptr to "pc". Hmm?

		E.